Structure, Reactivity and Mechanism
major exception being alkyl groups which are electron-donating.*
Though the effect is quantitatively rather small, it is responsible for
the increase in basicity that results when one of the hydrogen atoms
of ammonia is replaced by an alkyl group (p. 49), and, in part at any
rate, for the readier substitution of the aromatic nucleus in toluene
than in benzene. Several suggestions have been made to account for the
electron-donating abilities of CH 3 , CH 2 R, CHR 2 and CR 3 , none of
which is wholly convincing and the matter can be said to be unsettled.
All inductive effects are permanent polarisations in the ground
state of the molecule and are therefore manifested in its physical
properties, for example, its dipole moment. •
(ii) Mesomeric or conjugative effect
This is essentially a further statement of the electron redistribution
that can take place in unsaturated and especially in conjugated
systems via their w orbitals. An example is the carbonyl group (p. 158)
whose properties are not entirely satisfactorily represented by the
classical formulation (XVII), sor by the extreme polar structure
(XVIII), that may be derived from it by an electron shift as shown:* >c—o >c—o•» (XVII) (XVIII) (XIX)The actual structure is somewhere in between, i.e. a resonance hybrid'
of which the above are the canonical forms, perhaps best represented
by (XIX) in which the «• electrons are drawn preferentially towards
oxygen rather than carbon. If the carbonyl group is conjugated with
a ^>C=C<^ bond, the above polarisation can be transmitted further
via then-electrons:.Me—CH =?CH^CH==^ Me-CH—CH=CH—OMe— CH—CH—CH—O(XX)
* The metal atoms in, for example, lithium alkyls and Grignard reagents,
both of which compounds are largely covalent, are also electron-donating, leading
to negatively polarised carbon atoms in each case: R Li and R —«- Mg- Hal
(cf. p. 170).
( •
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